While Mr. Resig isn't adamantly against "language abstractions" - he notes many of their benefits - his counterpoint is summed up in this paragraph:
This is becoming a popular fallacy in programming language circles; treating Joel Spolsky's "Law of Leaky Abstractions" as if it were an actual law.
Let's examine the metaphor of the "leak". In plumbing, a leak is a hole in a pipe where water gets out. Joel has noticed that every pipe has a hole in it, and therefore all pipes are leaky.
But that's not quite accurate. There's another hole in pipes where water gets out: it's called the "faucet", and without that part, the rest of the pipe is pretty useless. To say that a pipe whose faucet is turned on is "leaky" is somewhat misleading, just as it's misleading to say that an abstraction that propagates errors in its lower levels is misleading. Joel's entire original essay is based on a subtle (and, I suspect, intentional) misunderstanding of TCP: the error conditions that result from failures in the lower level, unreliable packet delivery mechanism are not leaks in the abstraction, they are very carefully specified and thoroughly documented. They are part of the abstraction. The abstraction of TCP does not try to pretend that connections are never broken, it just provides a unified idea of a "broken connection" that is clearly specified so you don't need to understand the five million ways that packet delivery can go wrong.
Put more simply: there are abstractions which do not leak. The example that Joel provides is one of them: TCP is a comprehensive abstraction.
Then there are abstractions which really do leak. Every object-relational mapper that provides a facility where you need to directly execute SQL, for example, is leaking the SQL through the abstraction. Every web templating framework where you can directly generate strings is leaky: the browser speaks DOM, and if you're generating strings, then bytes are leaking through the abstraction.
In the case of UNIX, you are gaining none of the benefit of learning the PDP-11 instruction set. When a bug in the C compiler occurs (and it will occur - just as it's bound to occur in any compiler) what resources do you have, as a developer, to correct the problem? If you've learned nothing about PDP-11 assembler then you stand no chance in trying to repair, or work around, the issue.
So, for those of you who work on UNIX-like operating systems using that fancy "C" machine-code abstraction: how much PDP-11 assembler have you written recently?